This invention relates to a battery charging device for charging a lithium ion cell or a nickel cadmium cell, a battery pack having secondary cells, such as lithium ion cells or nickel cadmium cells, a battery charging method in case of charging plural cells, and a battery evaluating device for evaluating the performance of a cell set made up of plural series-connected cells.
Rechargeable cells or secondary cells, such as lithium ion cells or nickel cadmium cells, are nowadays known. These secondary cells are charged by a battery charging device shown for example in FIG. 1.
With the battery charging device, shown in FIG. 1, a charging positive terminal 51 is connected via a switch 53 to a positive electrode of a dc current source 50. A charging negative electrode 52 is connected to a negative terminal of the dc current source 50. Three secondary cells 54, for example, are connected in series across the charging positive electrode 51 and the charging negative electrode 52. Three discharging circuits, each comprised of a resistor 55 and a switch 56, are connected in parallel with the secondary cells 54 across the charging electrodes 51 and 52.
With the above battery charging device, the secondary cells 54 start to be charged on turning the switch 53 on. On starting the charging, the switches 56 of the discharging circuits are turned off.
If the secondary cells 54, thus connected in series with one another, are charged, the cells are charged to variable charging states, since the cells 54 are liable to variations in capacities.
The conventional practice for combatting such inconvenience has been to detect the voltages of the three secondary cells 54 and to turn on the switch 56 of the secondary cell 54 charged at the highest charging rate and the switch 56 of the secondary cell 54 charged at the second highest charging rate in case of occurrence of variations in the charging states of the three cells. This discharges electricity so far stored in the secondary cell 54 charged at the highest charging rate and in the secondary cell 54 charged at the second highest charging rate via the resistors 55.
When discharging of the secondary cell 54 charged at the highest charging rate and discharging of the secondary cell 54 charged at the second highest charging rate proceed and voltage values of these secondary cells 54 become equal to the voltage value of the secondary cell 54 charged at the lowest charging rate, the switches 56 of the discharging circuits which have been turned on are turned off, thus re-initiating charging.
With the above-described conventional battery charging device, charging and discharging are repeated by the on/off operation of the switch 58 for uniformly charging the secondary cells.
However, if the cells are charged with repetition of alternate charging and discharging operations, the totality of the cells are charged for conformity to the charging state of the secondary cell charged at the lowest charging rate, thus necessitating excessive time until full charging of the secondary cells is achieved.
In addition, battery charging is perpetually beset with the problem of excess charging. Such excess charging imposes load on the circuits of the batteries and the charging circuits, thus affecting safety due to heat evolution or breakage. Thus a demand has been raised for a battery charging device whereby excess charging may be eliminated to permit charging in safety.
On the other hand, the battery performance is evaluated by repeating a cycle of charging, dwell, discharging and dwell a plural number of times. However, sufficient quality evaluation has not been achieved with the battery being in use because the charging, dwell and discharging occur in an irregular manner. Besides, with a set of cells made up of plural series-connected cells, charging needs to be terminated at a time point when the cell having the least capacity is fully charged, such that it is not possible to evaluate the cells taking into account the variations in cell capacity from cell to cell and hence it is not possible to evaluate the true cell performance as a set of cells.